Genetic Variants Found that Influence the Odds of Twins

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Are there twins run in your family? There are a lot of “old wives tales” that have been passed around about what indicates that a pregnant women is having twins. None of those have any factual basis. A scientific study found that there are two genetic variants that actually do influence the odds of a woman having non-identical twins.

There are a lot of places on the internet in which people shared “old wives tales” that supposedly indicate that a pregnant woman is having twins. Maybe you have heard some of these before:

* A craving for yams

* Twins skip a generation

* Twins are caused by the husband’s side of the family

* Having stronger than typical pregnancy symptoms at the start of a pregnancy

* Twins “run in the family”

* A huge “baby bump”

None of the “old wives tales” about twins are factually correct. A study that was published in the American Journal of Human Genetics published findings about twins that the authors say have “important implications for human fertility, including improved outcome prediction and novel avenues of fertility treatment.”

For the study, researchers conducted a genome-wide association study using data from 1,980 mothers of natural fraternal twins and additional data from about 13,000 mothers or non-twins from cohorts in the United States, Australia, and the Netherlands. The Icelandic data was assembled by DeCODE. The researchers also used data from 23andMe’s study on the onset of puberty in women and another 23andMe study about PCOS.

One of the genetic variants the researchers found is located near the FSHB gene. That gene is one that encodes a portion of the follicle-stimulating hormone (FSH). The variant was associated with higher SH levels in women, an earlier age at menarche and menopause, and earlier age at first child and last child, an increased number of children, and a decreased risk for PCOS.

The other genetic variant is located in a gene called SMAD3. The authors of the study speculate that this gene might influence how the ovaries respond to the FSH hormone.

There is about a 1-to-4 percent of babies born that are fraternal, or dizygotic, twins. These twins might be of the same gender or different genders. A woman with one of the genetic variants that the researchers identified in their study increases her odds of having dizygotic twins by 9-18 percent and up to 27 percent if she has each of the two variants.

It should be noted that the genetic variants that the researchers found only relate to the odds of having dizygotic twins. Fraternal twins share the same amount of genetics as do regular siblings. So far, research has not shown that having identical twins is influenced by genetics.